1. Field of the Invention
This patent specification relates to a method and apparatus for image forming, and more particularly to a method and apparatus such as a copier, a printer, and a facsimile used for image forming in accordance with an electrographic method, capable of effectively controlling image forming process conditions.
2. Discussion of the Related Art
In a typical electrographic image forming apparatus such as a copier, a printer, and a facsimile, performances and characteristics of supplies such as a development agent and a photoconductor used in the apparatus vary over time or by environmental factors. Therefore, image forming process conditions should be appropriately controlled and adjusted in response to such variations in the performances and characteristics of the supplies so as to stably maintain image quality.
As is disclosed in Japanese Laid-Open patent publication no. 2002-108141, for example, in a control operation of the image forming process conditions (hereinafter referred to simply as condition control), variable factors are detected by a sensor and the like, and a feedback operation is performed so that conditions of image-formation-related units, such as a charging voltage level and a toner supply amount, are optimized. In this detection of the variable factors, an error range increases if the detection is based on data obtained at one moment of time. The detection thus should be based on a plurality of data values obtained at predetermined time intervals. As a result, the detection takes a certain amount of time.
Further, since the image-formation-related units are operated during the condition control, an image forming operation is suspended during the condition control. Therefore, an image output operation such as output of printed or photocopied sheets (hereinafter referred to as a job) is not accepted during the condition control. This results in down time of the image forming apparatus, causing dissatisfaction among users. In recent years, this dissatisfaction among users has been increasing with the spread of color image forming apparatuses. This is because a color image forming apparatus needs to execute the condition control for each of image forming units of four different colors black (K), cyan (C), yellow (Y), and magenta (M), taking four times as long for the condition control compared with a monochrome image forming apparatus in which only one image forming unit of the black color goes through the condition control. If the image forming process conditions are not controlled, the users are saved from waiting, but image quality is deteriorated.
In light of the above, according to Japanese Laid-Open patent publication no. 2002-108141, the job is performed even during the condition control. Since the condition control is discontinued, however, the condition control should be executed again from the beginning. As a result, although inconvenience for the users may be temporarily avoided, if a relatively long condition control is discontinued and then executed again from the beginning, longer time is taken in total for completing the condition control than for completing a typical condition control generally used. The patent publication also discloses a method of resuming the condition control starting from a discontinued phase of the condition control. According to the method, however, a particular discontinued phase of the condition control is not recognized, and thus a relatively long phase tends to be discontinued and re-executed in the resumed condition control. As a result, a longer time is taken in total for completing the condition control than for completing the typical condition control, as in the above case.
On the other hand, Japanese Laid-Open patent publication no. 2003-091109, for example, discloses a method of suspending the condition control during a job or when it is highly possible that the job is being performed. If the condition control continues to be suspended even after it becomes necessary to execute the condition control; however, the image quality eventually deteriorates. Therefore, the job should be discontinued at some point in time to execute the condition control.
In light of the above, according to Japanese Laid-Open patent publication no. 2002-132097, for example, a decision of whether or not to execute the condition control is left up to users. In this case, a user presses a predetermined button provided on the image forming apparatus depending on the decision. This method, however, is effective only when the user is in the vicinity of the image forming apparatus, as in a case of a copier.
Further, according to Japanese Laid-Open patent publication no. 10-114128, for example, when it becomes necessary, during an ongoing job, to execute the condition control, a decision of whether or not to discontinue the job is made in consideration of the type of the job. Thereafter, a predetermined action is taken, such as not discontinuing the ongoing job, immediately discontinuing the ongoing job, and discontinuing the ongoing job after output of a predetermined number of sheets. Furthermore, Japanese Laid-Open patent publication no. 2002-229278, for example, uses a particular accumulated number of output sheets as a condition for starting the condition control. Both of the above methods address responses to be made when the execution of the control operation becomes necessary during an ongoing job, but not the responses to be made when a request for a job is received during an ongoing condition control.
In light of the above, Japanese Laid-Open patent publication no. 9-314903, for example, discloses an image forming apparatus which executes a condition control formed by combining a plurality of phases each designed to complete in relatively short time. According to this image forming apparatus, when a print output signal is issued during the condition control, a phase of the condition control being performed at the issuance of the signal is completed. Then, a phase scheduled to be performed after completion of the phase is suspended to preferentially perform a print output operation. As a result, the image forming apparatus can perform an operation requested by a user, without keeping the user waiting long. This method, however, has an open question of how the discontinued condition control should be resumed to stably maintain image quality.
FIG. 1 is a graph indicating a relationship between a waiting time for users and a degree of dissatisfaction the users have toward the waiting time (hereinafter referred to as dissatisfaction degree). In this graph, the horizontal axis represents the waiting time for users, and the vertical axis represents the dissatisfaction degree. The present inventors conducted research to find a time range allowable for adjustment of the image forming process conditions. From a result of the research, the relationship between the waiting time for users and the dissatisfaction degree can be expressed as in a sigmoid function D=1/(1+EXP(−k2*Ln(t/k1)))*100(%), wherein D indicates the dissatisfaction degree, EXP indicates an exponential function, k2 indicates a constant indicating steepness of a rising edge of the sigmoid function, Ln indicates a natural logarithm, t indicates time, and k1 indicates a time scale constant. The parameter k1 ranges from 9 to 15 seconds and the parameter k2 ranges from 2 to 3. These parameters vary depending on such factors as the type of image forming apparatus (i.e., printing speed of the image forming apparatus) and the way the image forming apparatus is used. It is observed from the graph of FIG. 1 that most of research subjects do not have dissatisfaction toward a waiting time of within approximately four to five seconds, a half of the research subjects have dissatisfaction toward a waiting of approximately nine to fifteen seconds, and most of the research subjects have dissatisfaction toward a waiting time exceeding approximately thirty seconds.